1. Field of the Invention
The present invention relates to a circuit that is capable of correcting a power factor in a small-sized power supply device for a Light-Emitting Diode (LED) lamp, a compact fluorescent lamp or a small-sized adapter.
2. Description of the Related Art
Since an LED lamp, a compact fluorescent lamp and a small-sized adaptor are small in size, small-sized power supply devices are used therein. A conventional power supply device is configured as shown in FIG. 1, so that AC current is supplied to a bridge diode circuit and then forms pulsating current, and the pulsating current is brought into approximation with DC current by an electrolytic capacitor and is then supplied to load.
Meanwhile, since the conventional technology simply uses an electrolytic capacitor, charging rapidly starts near the peak value of input voltage and then discharging is rapidly performed, as shown in the second graph of FIG. 3. Therefore, the time period over which input current flows is short, so that the input current flows only during a part of the interval of the waveform of the input voltage, with the result that the power factor decreases and therefore a loss occurs on a transmission line that supplies power.
In order to mitigate this problem, a power supply device having a Power Factor Correction (PFC) circuit should be applied. However, an LED lamp, a compact fluorescent lamp and a small-sized adaptor have internal spaces that are too small to accommodate such a circuit, and also an increase in cost occurs due to the addition of such a PFC circuit. Thus, it is difficult to manufacture a power supply device, to which a PFC circuit has been applied, at low cost and in small size, and therefore such a PFC circuit is not applied in many cases, thereby resulting in energy being wasted.
Meanwhile, such PFC circuits may be classified into active PFC circuits and passive PFC circuits. A representative passive PFC circuit uses a resonance circuit (LC) method in which an inductor L corresponding to an electrolytic capacitor C is used, as shown in FIG. 2. In this method, the power factor is improved by the inductor which compensates for the phase difference generated by the capacitor, compared to that shown in the second graph of FIG. 3, as shown in the third graph of FIG. 3 (that is, the phase lead generated by the capacitor is compensated for by the phase lead of the inductor). This passive method can normally improve the power factor up to 0.8.
However, since AC current has a low frequency in a range of 50 Hz to 60 Hz and a low frequency inductor is large and heavy, it is difficult to apply such a passive PFC circuit to a small-sized power supply device.
Furthermore, although an active PFC circuit can achieve a high power factor using a switching method, it has the disadvantages of being expensive because of the complication of the circuit thereof and requiring a countermeasure for EMI due to the generation of harmonics resulting from switching. Accordingly, it is difficult to apply such an active PFC circuit to a small-sized power supply device having a small internal space and requiring low cost manufacturing.